In a conventional condensation polymerization system the reactors are separate vessels; and each of the vessels has its own vacuum system with a direct-contact spray condenser. Frequently these systems experience problems as by-products of the reaction and excess reactants are removed from the melt. Some monomer and low-molecular-weight polymer, which have a high propensity for solidification, foul or plug condensers and vacuum equipment. This fouling is one of the most frequent reasons polymer plants lose annual production capacity.
Fundamentally, components have solubility limits of concentration in the fluids that are present in vapor-handling equipment. Fouling can occur whenever the concentration of a component exceeds its solubility limit. When these limits are exceeded in the laboratory or full-scale production plants, one observes cloudy condensates; and fouling of equipment is likely. Over time, this fouling causes operational problems that must be dealt with. Often, production plants must interrupt normal operations to handle the solid build-up. They lose valuable production capacity during that time.
To alleviate the fouling in vapor handling equipment, several approaches have been developed. First, equipment is jacketed to raise the surface temperatures. In theory, higher surface temperatures allow higher concentrations to be present before solubility limits are exceeded. This is only marginally effective, however, because the solvent tends to evaporate. Where the mixtures are somewhat stagnant, this evaporation enriches the mixture in solids-forming ingredients, concentrations rise above solubility limits, and solids adhere to the surfaces.
Design and operation of the condenser to handle relatively high levels of solids-forming material is another approach. Scraped-surface condensers are common now in many polymer plants. These devices have moving mechanical parts that remove solids as they build up on surfaces.
A solution to plugging in ejectors is described in U.S. Pat. No. 3,468,849. Instead of using steam as the motive fluid for an ejector, they use a solvent that tends to dissolve the solids-forming materials. For polyethylene terephthalate (PET), the motive fluid is ethylene glycol vapor. Since ethylene glycol is a primary reactant for the PET reaction and a good solvent, it is an ideal fluid for the ejectors.
The apparatus disclosed in U.S. Pat. No. 3,468,849 has several independent reaction vessels or chambers. Since a single ejector cannot practically be designed to achieve the very low pressures required by the process, each reaction chamber has its own multistage vacuum system to compress the inerts from the given reaction chamber.
However, this configuration has been found to be undesirable in several respects. The capital cost of an additional condenser and an additional vacuum system and the auxiliary equipment required for operation of the condenser and vacuum system is quite large. In addition to raising the capital cost per pound of polymer produced, the extra items diminish the overall reliability of the polymer plant. Moreover, each vacuum system has its own motive fluid requirements. Thus, additional vacuum systems greatly increase the amount of motive fluid which must be treated to remove environmental hazards. Accordingly, there remains a need in the polymer production art for a simplified system to condense vapor discharged from the reaction chambers and to generate the required vacuum levels for the respective reaction chambers.